stmmac: dwmac-loongson: fix missing pci_disable_msi() while module exiting

pci_enable_msi() has been called in loongson_dwmac_probe(),
so pci_disable_msi() needs be called in remove path and error
path of probe().

Fixes: 30bba69d

 ("stmmac: pci: Add dwmac support for Loongson")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>